p-Type transparent conducting oxides and methods for preparation

ABSTRACT

A facile, low temperature and low pressure method for the preparation of a wide range of phase pure ABO 2  compositions.

This application claims priority benefit from provisional applicationSer. No. 60/281,211 filed on Apr. 3, 2001, the entirety of which isincorporated herein by reference.

The United States Government has certain rights to this inventionpursuant to Grant No. AAD-9-18668-05 from the Department of Energy andGrant No. DMR-0076097 from the National Science Foundation, both toNorthwestern University.

BACKGROUND OF THE INVENTION

This invention relates generally to transparent conducting oxides, andmore particularly, to such compositions and related structures havingp-type conductivities and methods for their preparation underhydrothermal reaction conditions.

Transparent conducting oxides (TCOs) are degenerate wide band-gapsemiconductors with conductivities comparable to metals, but aretransparent over the visible and IR regions. Currently, the best knownand industrially useful TCOs are doped ZnO, SnO₂ and In₂O₃, all of whichare n-type semiconductors. For example, in thin film forms, Sn-dopedindium oxide has n-type conductivity on the order of 10³ S/cm and anaverage transmittance higher than 85% in the visible light range. Bycomparison, in thin film form, the p-type conductivity of CuAlO₂ isabout 1 S/cm and about 10⁻³ S/cm in bulk form. (H. Kawazoe, M. Yasukawa,H. Hyodo, M. Kurita, H. Yanagi and H. Hosono, Nature, 389, 939–942(1997). p-Type Electrical Conduction in Transparent Thin Films ofCuAlO₂. F. A. Benko and F. P. Koffyberg, J. Phys. Chem. Solids, 45, 1,57–59, (1984). Opto-electronic properties of CuAlO₂.)

Many ternary oxides with an A^(I)B^(III)O₂ composition adopt thedelafossite (CuFeO₂) structure, where A is either Cu, Pd, Pt or Ag and Bis a trivalent metal with 0.53<r(B_(VI) ³⁺)<1.09 Å. Thesedelafossite-type oxides comprise a rich family of compounds withinteresting luminescence properties and applications in areas ofcatalysis or electrocatalysis. Until recently, only SrCu₂O₂ andnitrogen-doped ZnO were the only known p-type TCOs. The recent discoveryof simultaneous p-type conductivity and transparency in CuAlO₂ hasheightened interest in CuMO₂ compounds and in particular those havingdelafossite structures. Owing to this unique dual property, TCOs findvarious technological applications in solar cells, optoelectronicmaterials, energy-efficient windows, gas sensors and flat paneldisplays, among others. The discovery of new p-type TCOs will open upnew application possibilities that are simply not feasible with unipolarn-type materials alone.

To date, most of the bulk CuAlO₂ syntheses correspond to direct- orcation exchange reactions in the solid phase. [B. U. Köhler and M.Jansen, Z. Anorg. Allg. Chem., 543, 73–80 (1986). Darstellung undStrukturdaten von Delafossiten CuMO₂ (M=Al, Ga, Sc, Y); T. Ishiguro, A.Kitazawa, N. Mizutani and M. Kato, J. Solid State Chem., 40, 170–174(1981). Single-crystal growth and crystal structure refinement ofCuAlO₂. >>; H. Hahn and C. Lorent, Z. Anorg. Allg. Chem., 279, 281(1955); B. Köhler and M. Jansen, Z. Krist., 129, 259 (1983); J. P.Doumerc, A. Amar, A. Wichainchai, M. Pouchard and P. Hagenmuller, J.Phys. Chem. Solids, 48, 1, 37–43 (1987). Sur Quelques Nouveaux Compośesde Structure de type Delafossite.] Based on the work of Croft et al. onAgFeO₂ [W. J. Croft, N. C. Tombs and R. E. England, Acta Chryst., 17,313 (1964). Crystallographic data for pure Crystalline Silver Ferrite.],Shannon et al. [R. D. Shannon, D. B. Rogers and C. T. Prewitt., Inorg.Chem., 10, 4, 713–727 (1971). Chemistry of Noble Metal Oxides. I.Syntheses and properties of ABO₂ Delafossite compounds. II. Crystalstructures of PtCoO₂, PdCoO₂, CuFeO₂ and AgFeO₂. III. Electricaltransport properties and crystal chemistry of ABO₂ compounds with thedelafossite structure.] reported the first hydrothermal synthesis ofCuAlO₂ as well as other ABO₂ compounds, using a thin-walled platinumtube at 500° C. with 3000 atm of externally applied pressure. However,in addition to the reaction conditions (high temperature and pressure,with prolonged reaction times), a limitation of this technique is thatmany of the delafossite-type compounds could not be isolated as singlephases, without a separate isolation or leaching procedure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Final percentage of CuAlO₂ obtained by direct comparison withPC-PDF standard and corroborated by the mass percentage calculated byRietveld analysis.

FIG. 2: Observed (o), calculated (line), and difference (bottom line)X-ray diffraction profiles for 3R—CuAlO₂. Calculated positions, thosereflections that are marked with vertical bars, and intensities matchthose of JCPDS #77-2493; refined structural parameters (α=2.8617(2)Å,c=16.9407(8)Å, z(O)=0.1089(2)) match single-crystal data. To confirmphase formation and purity, powder X-ray diffraction (XRD) data for eachsample were collected every 0.05° for 10°<2θ<70° on a Rigakudiffractometer with Ni-filtered Cu Kα radiation. For phase-pure samples,data were collected every 0.02° for 10°<2θ<110°. Rietveld refinementswere performed using the FULLPROF software program. (Rodriguez-Carvajal,J. Abstracts of the Satellite Meeting on Powder Diffraction of the XVCongress of the IUCr; Toulouse, France, 1990; p 127).

FIG. 3: TGA curves under reducing conditions (H₂/N₂) of CuAlO₂ samples.Solid State Synthesis sample (SSS) and Hydrothermal Synthesis sample(HS).

FIG. 4: CuAlO₂ Scanning Electron Microscopy images, comparing a presentcomposition (4A) and one prepared according to the prior art (4B).

FIG. 5 Temperature dependence of normalized electrical conductivity forthe HS CuAlO₂ sample.

FIG. 6 Power X-ray diffraction patterns for (A) hydrothermal synthesisof Cu Al_(1-n)Ga_(n)O₂ solid solution, where n=0 (bottom), n=0.50(middle), and n=1.0 (top); and (B) 1000° C. solid-state synthesis in airof CuAlO₂ (bottom), a two-phase product consisting of a 1:1 mixture ofCuAlO₂ and CuGaO₂ (middle), and CuGaO₂ (top).

SUMMARY OF THE INVENTION

In light of the foregoing, it is an object of the present invention toprovide transparent conducting oxides, compositions and/or methods fortheir preparation, thereby overcoming various deficiencies andshortcomings of the prior art, including those outlined above. It willbe understood by those skilled in the art that one or more aspects ofthis invention can meet certain objectives, while one or more otheraspects can meet certain other objectives. Each objective may not applyequally, in all its respects, to every aspect of this invention. Assuch, the following objects can be viewed in the alternative withrespect to any one aspect of this invention.

It is an object of the present invention to provide moderate reactionconditions for the facile hydrothermal synthesis of transparentconducting oxides, including but not limited to the systems describedherein by example and/or illustrating the present methodology.

It can also be an object of the present invention to provide ABO₂compounds and/or materials with phase purities not otherwise achievablethrough hydrothermal methods of the prior art.

It can also be an object of the present invention to provide a generalmethodology whereby a variety of transparent conducting oxides,including but not limited to those having a CuMO₂ formula, can bedesigned and/or tailored through isovalent cationic substitution, suchoxides further providing a variety of preconceived optical and/orelectrical properties.

It can also be an object of the present invention to provide one or moreABO₂ compounds and/or materials, including those having a CuMO₂molecular formula, such compounds and/or materials preferably having adelafossite-type structure with oxygen intercalation.

It can also be an object of the present invention to provide, inaccordance with one or more of the preceding objectives, a p-type TCOcompound and/or material for use and/or integration into a variety ofdiodes, transistors, and p-n heterojunctions, as well as for use inconjunction with thin film sputtering targets.

Other objects, features, benefits and advantages of the presentinvention will be apparent from this summary and its descriptions ofvarious preferred embodiments, and will be readily apparent to thoseskilled in the art having knowledge of various transparent conductingoxides and related synthetic techniques. Such objects, features,benefits and advantages will be apparent from the above as taken intoconjunction with the accompanying examples, data, figures and allreasonable inferences to be drawn therefrom.

In part, the present invention is a hydrothermal method for thepreparation of oxides having the formula ABO₂. Such a method avoids theextreme reaction conditions of the prior art, using commerciallyavailable metals and metal oxides reacting under moderate temperaturesand without application of external pressures. As illustrated elsewhereherein, this methodology can be considered in view of the reactionformula and stoichiometries shown in equation (1), below.$\begin{matrix}{{{x\mspace{11mu} A^{II}O} + {{\left( {1 - x} \right)/2}\mspace{11mu} A_{2}^{I}O} + {{x/3}\mspace{11mu} B} + {{\left( {3 - x} \right)/6}\mspace{11mu} B_{2}O_{3}}}\overset{NaOH}{\rightarrow}{{ABO}_{2}\left( {{{where}\mspace{14mu} 0} \leq x \leq 1} \right)}} & (1)\end{matrix}$

In accordance with the methodology and related aspects of thisinvention, a wide variety of oxide compositions can be preparedhydrothermally. Such a method for preparation can include (1) providing,mixing and/or placement of the desired metallic (B-site) and oxide(A-site and B-site) reagents and/or starting materials, under basicconditions, in a suitable reaction vessel, such a vessel conducive tothe introduction of water therein; (2) heating the reaction medium to afirst temperature and/or sufficient to initiate reaction of thereagents/starting materials; and (3) maintaining the reaction at atemperature, preferably a second higher temperature, sufficient toprepare the oxide product. Such temperatures are less than and/orsubstantially less than about 500° C. Likewise, reaction pressures areless than and/or substantially less than about 3000 atmospheres.Generally, the pressure and temperature parameters of the presentmethodology are within ranges considerably less than those referenced inconjunction with hydrothermal methods of the prior art. Phase pure solidoxide product can be isolated upon incremental cooling of the reactionmedium, to room temperature. Other aspects relating to the compositionsand/or methods of this invention are as elsewhere described herein.Autoclave/bomb and related equipment considerations will be well knownto those skilled in the art and made aware of this invention.

In accordance with the broad methodology of this invention, and withreference to equation (1) above, a range of synthetic transformationscan be achieved. With consideration of the stoichiometric relationshipof equation (1), the relative amount of each reagent or startingmaterial can vary with the value of x. Likewise, the value of x can bevaried between 0 and 1, as would be understood by those skilled in theart made aware of this invention, to determine optimal reactionconditions for each A-B system en route to the desired ABO₂ composition.Several examples of this invention, as provided below, illustrate thescope of this invention in the context of copper aluminate syntheses.

Accordingly, the present invention also includes phase pure ABO₂compositions, of the type as can be prepared using the methodologyoutlined above. In preferred embodiments, A is a monovalent cation and Bis a trivalent cation, each of which as provided in the ABO₂material/compound in accordance with and corresponding to the respectivecationic or metallic reactants shown in equation (1), above. Othervalence and stoichiometric relationships are possible, but theaforementioned mono/trivalent relationship is especially useful by wayof providing delafossite-type structures together with the optical andelectrical properties resulting therefrom. Regardless, the A-site metalcan include Cu, Pd, Pt and Ag. In a broader sense, d¹⁰ cations arepreferred and most likely to provide transparent conductors. B-sitemetals are preferably Al, Ga, Sc and/or In, and can otherwise beselected from the lanthanide (Ln) series. In particular and withoutlimitation, the present invention contemplates CuMO₂ compounds and/ormaterials having a delafossite-like structure, where M is Al, Ga, In, Scor Ln (lanthanides). As inferred above, other non-delafossite-typespecies can be prepared as described herein from the correspondingcations/metals for use in catalytic and electrocatalytic applications.

In part, the present invention includes a more general hydrothermalmethod for the preparation of novel solid solutions previouslyunobtainable via other synthetic methods. Such solid solutions,compounds, compositions and/or materials can be prepared, in accordancewith this invention via a straight-forward extension of the methodologyillustrated by equation (1), as shown in equation (2), below.1−m(xA ^(II)O+(1−x)/2A ^(I) ₂O)+m(xA′ ^(II)O+(1−x)/2A′ ₂^(I)O)+1−n(x/3B+(3−x)/6B₂O₃)+n(x/3B′+(3−x)/6B′₂O₃)→A _(1-m) A′_(m)B_(1-n)B′_(n)O₂  (2)

-   -   (where 0≦m≦1 and 0≦n≦1)

As discussed above, various combinations of mono- and trivalentcationic/metal species can be utilized to provide a wide variety ofsolid solutions, compositions, materials and the like. In particular,the optical (e.g., transparency) and/or electrical (e.g., p-typeconductivity) properties of each can be tailored or designed throughcationic/metal choice and incorporation of such species into a syntheticsequence in accordance with the chemical and/or stoichiometricrelationships described herein. Reference is made to example 11, below,and variations in the relative stoichiometries of Al and Ga to provide arange of such compositions with varying optical and electricalproperties.

In part, the present invention can also provide a method for oxygenintercalation and/or of preparing transparent conducting oxides havingenhanced total oxygen content, such content enhanced as compared tomethods, procedures and/or compositions of the prior art. As describedmore fully below and as would be understood by those skilled in the art,greater total oxygen content (O_(2+δ)) provides a favorable effect onthe resulting electronic properties. Without restriction or limitation,this and other aspects of the present invention can be considered withanalogy to one or more principles underlying Vegard's Law, inparticular, the relationship of unit cell size and interstitialdimensions. Larger B-site cations afford increased interstitial spacingand potential for oxygen intercalation. Accordingly, the compositionsand materials for this invention provide a range for increased oxygencontents (O_(2+δ)), the extent of which as can be determined by choiceof A- and/or B-site cations and corresponding unit cell design.

In part, the present invention is also a method of using a B-site metal,corresponding to the present reaction schematic, to facilitatepreparation of the ABO₂ compounds of this invention. In particular, aneutral B-site metal is utilized as a reducing agent for thecorresponding A-site metal cation. Such reduction facilitatesdissolution of the corresponding cationic reactant en route to the oxideproduct. Such a redox relationship is achieved under basic conditions.As applicable to other aspects of the inventive methodology, suchconditions are preferentially achieved using sodium hydroxide. Such areagent is chosen primarily for reasons of economy. However, therequisite basic conditions can also be provided through use of otherreagents known to those skilled in the art.

As discussed above, the present invention includes a single step,hydrothermal synthetic process whereby polycrystalline samples of anydelafossite-like (ABO₂) material, can be synthesized with phase purityat low temperature and pressure. More particularly,A_(1-m)A′_(m)B_(1-n)B′_(n)O₂ solid solutions (where 0≦m≦1 and 0≦n≦1),including but is not limited to CuAl_(1-n), Ga_(n)O₂, previouslyunattainable by prior art methods, can now be synthesized. Although suchdelafossite compounds have long been known as have interestingluminescence properties and applications in catalysis andelectrocatalysis, they have become the subject of much renewed interestwith the recent discovery that CuAlO₂, for example, exhibits both p-typeconductivity and transparency. It is in this transparent conductingoxide arena, in particular, where this invention may find its mostuseful and productive applications. For instance, such delafossitessynthesized hydrothermally exhibit greater conductivities than hightemperature ceramic products of the prior art.

As a point of contrast, traditional ceramic methods typically requirehigh-energy input, in the form of increased temperature, and arediffusion limited, requiring several regrinding steps to increasehomogeneity and prolonged reaction times. The present hydrothermalmethods are especially desirable for preparing advanced materialsbecause such obstacles can easily be overcome. The present syntheticmethods typically involve temperatures no higher than about 210–215° C.,pressures of approximately 5–10 atm, and reaction times that are atleast one order of magnitude shorter than for analogous ceramicreactions. As mentioned above, the only other known prior arthydrothermal synthesis of delafossite materials requires temperaturesgreater than 500° C. and externally applied pressures of 3000 atm. Evenunder such extreme reaction conditions and unlike the present invention,phase purity could not be achieved for most products. Subsequentpurification steps are required.

With regard to one aspect of this invention, the use of a neutral B-sitemetal as a reducing agent rather than as a direct exchange reactant hasbeen shown to facilitate the dissolution of species otherwise difficultto dissolve, resulting in a significant increase product yield. WithCuBO₂ compounds, for example, these redox active species readily oxidizein basic solution while simultaneously reducing the aqueous Cu^(II)species to the desired Cu^(I) species. This redox step is more easilyaccomplished for copper than is the dissolution of a Cu^(I) speciesunder aqueous conditions alone, as evidenced in the Pourbaix diagram. Assuch, the redox component of the overall reaction appears to facilitatephase purity.

EXAMPLES OF THE INVENTION

The following non-limiting examples and data illustrate various aspectsand features relating to the compositions/materials and/or methods ofthe present invention, including the preparation of a variety of p-typetransparent conducting oxides as are available through the syntheticmethodology described herein. In comparison with the prior art, thepresent methods and compositions/materials provide results and datawhich are surprising, unexpected and contrary to the prior art. Whilethe utility of this invention is illustrated through the use andcharacterization of several compositions/materials and theirpreparation, it will be understood by those skilled in the art thatcomparable results are obtainable with various other methods,compositions and/or materials, as are commensurate with the scope ofthis invention.

Example 1a

Instead of using thin-walled platinum tubes under the harsh conditionsof Shannon et al, Teflon (fluoro(ethylene-propylene)) pouches wereemployed. The synthesis of this example and all related copper aluminatesyntheses were made starting with commercially available aluminum metaland copper and aluminum oxides. Appropriate amounts of these specieswere placed along with ground NaOH pellets in each Teflon pouch. Molefractions of the reactants were varied with x in accordance with thefollowing reaction: $\begin{matrix}{{{x\mspace{11mu}{CuO}} + {\frac{1 - x}{2}{Cu}_{2}O} + {\frac{x}{3}{Al}} + {\frac{3 - x}{6}{Al}_{2}O_{3}}}\overset{NaOH}{\rightarrow}{CuAlO}_{2}} & (3)\end{matrix}$The total mass was constant and equal to 0.8 g. The pouches were sealedand placed into a 125 ml autoclave filled with deionized water. Theautoclave was sealed and heated at 150° C. for 5 hrs and subsequently at215° C. for 48 hrs, followed by slow cooling to room temperature at 0.1°C./min. Afterwards, each pouch contained both solid products andapproximately 2.5 g of solution, owing to the permeability of the Teflonpouch and the basicity of the inside species. The Teflon pouches heatedabove 100° C. become permeable and an exchange reactions occurs betweenthe inside powder and the outside solution. This reaction is veryexothermic and appears to be responsible for the two-step heatingprocess. The first step at about 150° C. seems to initialize thereaction, and the second at about 215° C. completes it.

Time and temperature were optimized, and the best results were obtainedfor the conditions described above. The filtered and dried solidproducts were analyzed by powder X-ray diffraction (XRD), scanningelectron microscopy (SEM), differential thermal and thermogravimetricanalyses (DTA and TGA), and the filtrate by inductively coupled plasmaatomic emission spectroscopy (ICP).

Example 1b

More generally, delafossite-like materials were synthesized by placing0.2 g NaOH(s)) along with stoichiometric amounts (see example 1) of AO,A₂O, B, and B₂O₃ in an FEP (fluoro(ethylene-propylene)) Teflon pouch.The pouch was sealed and placed in a 125-ml Teflon-lined autoclave(Parr) filled with 80-ml of deionized water. The total contents of eachpouch were held constant to 0.8 g. The autoclave was sealed and firstheated to about 150° C. for 5 hours to allow H₂O to enter the permeablemembrane of the pouch and dissolve the NaOH(s). This was followed by anapproximate 210° C. step for 48 hours, with subsequent cooling to roomtemperature at 6° C./hr. Delafossite crystallites were recovered byfiltration.

Example 1c

Delafossite phases prepared in accordance with this invention, usingprocedures analogous to that described in example 1b, include CuAlO₂,CuGaO₂, CuFeO₂, CuLaO₂ and CuAl_(1-x)Ga_(x)O₂ (solid solution); andAgGaO₂, AgInO₂ and AgScO₂.

Example 1d

The diagram of this example plots the known size of an A-site cation(Cu, Pd, Pt, or Ag) vs. the known size of a B-site cation in ABO₂delafossites. The bullets or points on this diagram represent reportedbulk delafossite phases in the literature. (For example, the pointintersecting 0.80 Angstroms on the ordinate axis and 0.46 Angstroms onthe abscissa corresponds to the compound CuInO₂.)

The phases/compositions synthesized above are representative of the manyphases available using this invention. Owing to size effects which arewell-known, accepted and understood in the art, and having made CuAlO₂and CuLaO₂ delafossite structures, using this inventive methodology, anycopper-containing delafossite compound that has a B-site cation rangingin ionic radius dimension between aluminum and lanthanum can also bemade. This analogy can be applied in both the horizontal (to include Pt,Pd and Ag compositions) and vertical directions of the diagram. It isalso possible to synthesize previously unknown compounds and solidsolutions.

Example 1e

AgBO₂ compounds cannot be prepared using traditional high temperaturesolid-state techniques since Ag₂O decomposes at temperatures above ˜160°C. Palladium and platinum oxides also decompose at higher temperatures(˜600° C.). Previously reported syntheses of these materialsincorporated multi-step cation exchange reactions. The availability ofsuch materials (e.g., AgGaO₂, AgInO₂ and AgScO₂ and correspondingrepresentative Pd and Pt compositions) is a significant advance in thesynthesis of delafossite-like materials. Preliminary data indicatesthat, similar to CuAlO₂, hydrothermally synthesized silver-containingdelafossites have surprising and/or unexpected electrical conductivitiescompared to their high temperature or cation exchanged counterparts.Furthermore, as described elsewhere herein, a variety of AgBO₂ andAgB_(1-n)B_(n)O₂ compositions heretofore unavailable or unknown in theart can be prepared and/or are contemplated in the context of thisinvention.

Example 2

With reference to the present methodology, optimal basic conditionsappear to correspond to equimolar concentrations of NaOH and the A- orB-site species. It is important to note that these are only the initialconditions, since the basic conditions during the reaction are dynamic.As the quantity of water inside the pouches increases, and the speciesare solvated, the alkalinity of the solution decreases, ultimatelyleading to a slightly basic (pH≈8) solution at the end. Alkalineconditions aid to introduce the water into the vessel pouch, therebytransforming the initial oxides into hydroxides or oxy-hydroxides. Abalance of reaction conditions promotes and maintains an optimal amountof incoming water and the solubility of these metal species. Al₂O₃ andAlO(OH) are not problematic, because these species can be dissolvedeasily under either acidic or basic conditions. A factor in theformation of CuAlO₂ can be, however, the presence of any Cu^(I) speciesin solution. Both the original and revised Pourbaix diagrams at standardpressure indicate that, regardless of temperature, a Cu^(I) species canexist only in strongly acidic conditions. The hydrothermal formation ofCuAlO₂ clearly contradicts this notion. Although achieved in moreextreme regions of temperature and pressure, the previous results ofShannon, et al. further verify that at least one Cu^(I) species canexist in an alkaline solution.

Example 3

Results obtainable through the present invention can be explained,without limitation, to this and several preceding and subsequentexamples. According to reaction scheme (3) described above, manydifferent syntheses were tried with a variation of x. For x=0.00, thereaction is simply a dissolution and subsequent reaction of the Cu^(I)(Cu₂O or CuOH) and Al^(III) (Al₂O₃ or AlO(OH)/Al(OH)₃) species. For allother x values, the results can be explained with a redox reactioninvolving at least the Cu^(I)/Cu^(II) and Al⁰/Al^(III) redox-couples.The stoichiometric coefficients in this reaction are calculated on thebasis of the reduction of Cu^(II) to Cu^(I) compensated by the oxidationof Al⁰ to Al^(III). All of the stoichiometric reactions produce CuAlO₂along with some un-reacted copper species. The final CuAlO₂ phasepercentage obtained, as varies with x, is shown in FIG. 1. Starting atx=1.00, FIG. 1 shows that the formation of CuAlO₂ increases when xdecreases for all x>0.30. The amount of CuAlO₂ obtained stabilized overthe range from 0.00<x≦0.30. It is apparent from the observed decrease ofCuAlO₂ for the x=0.00 composition that the complete absence of Cu₂Oprecursor hinders the CuAlO₂ yield. On the other hand, using Cu₂O as theonly Cu precursor is even less efficient. The curve presented in FIG. 1indicates that for the various syntheses under different conditions, theoptimum results are observed in the 0.10≦x≦0.30 composition range.

Example 4

Without limitation to any one theory or mode of reaction, the results ofthe previous examples can be explained using two different mechanisms.The first one corroborates the results obtained by Shannon et al.,namely that, although difficult, Cu₂O can be dissolved in a basicsolution and can, along with Al₂O₃, produce 30–40% CuAlO₂. This is thecase for the x=0.00 composition. The second mechanism corresponds to aredox reaction between Cu^(I)/Cu^(II) and Al⁰/Al^(III), leading tovarious amounts of CuAlO₂ based on the composition. This mechanism isbelieved completely responsible for the x=1.00 composition. All of theintermediate compositions can be explained by a combination of these twomechanisms. For these cases, production of CuAlO₂ seemingly occurs bydirect dissolution of Cu^(I) and by reduction of Cu^(II) into Cu^(I) insolution. When x decreases from 1.00 to 0.00, the initial amount of Cu₂Oincreases leading to an increased amount of Cu^(I) species in solution,and thereby increasing the contribution of the first mechanism. Asolubility limit is encountered at high initial Cu₂O concentrations,since for all of the compositions with an initial x<0.30, the solutioncannot accept any more Cu^(I) species and therefore we witness aplateau. The other component of the final CuAlO₂ yield is a redoxreaction, where, in solution, the Al metal is oxidized into Al^(III) asthe Cu^(II) is reduced into Cu^(I). As x is increased, the predominantform of the initial copper species is CuO. The results show that atlower concentrations of initial redox species, the higher the Cu yieldinto CuAlO₂, and as this amount increases, (i.e. as x increases), theyield decreases to a final value of only 30% for x=1.00. Taking all ofthis into account facilitates understanding the curve relating to thesecond mechanism in FIG. 1. This second mechanism appears to exactgreater influence as x is increased, becoming the predominant mechanismfor x≧0.50.

Example 5

In order to illustrate the possible effect of these mechanisms,non-stoichiometric reactions were tried, either with an excess ofaluminum reagents or with a copper deficiency for the 0.10<x<0.30compositions. The excess aluminum species do not affect the CuAlO₂yield, confirming that they are not the limiting reagents in terms ofsolubility. On the other hand, when the initial amount of the copperspecies are decreased by half, phase pure CuAlO₂ was finally obtained.Using, for the purpose of this example, the results from the x=0.33reaction this phenomenon can be elucidated: one-half of the initialcopper species reacts to produce CuAlO₂, while the other half remainsunchanged. Only by removing the unreacted half of the initial copperspecies can phase purity be obtained with 100% yield in the copperspecies. ICP analysis reveals that the remaining half of all aluminumspecies is still in solution.

Example 6

The process or mechanism by which Cu^(I) species are generated is notcompletely understood, but several comments can be made. As evidenced,the synthetic methodology of this invention appears to be a complexcombination of solubility factors and redox reactions. In addition tothe Cu^(I)/Cu^(II) and Al/A^(III) couples, the H⁺/H₂ pair can also takepart in the reaction by assisting first in the oxidation and hydrolysisof the aluminum metal into Al(OH)₃ and furthermore into NaAlO₂ with thesodium hydroxide, and second, in the reduction of Cu^(II) into Cu^(I).The mechanism of the direct dissolution of Cu^(I), under theseconditions, is not explained, and as we know, no other example isreported in the scientific literature. The second mechanism is not sosurprising, and is comparable to the common test for glucose in urinewhich corresponds to the following reduction of Cu^(II) into Cu^(I):RCHO_((aq))+2Cu²⁺ _((aq))+5OH⁻→4 RCOO⁻ _((aq))+Cu₂O_((s))+3H₂O_((I)).Finally, without limitation, it should be noted that thedisproportionation of Cu^(I) into Cu^(II), and Cu⁰ was not observed inthe reactions undertaken to demonstrate this invention.

Example 7

Hydrothermally obtained CuAlO₂ was refined by a Rietveld analysis [H. M.Rietveld, Acta. Cryst., 22, 151–2 (1967). Line Profiles of NeutronPowder-Diffraction Peaks for Structure Refinement. H. M. Rietveld, J.Appl. Cryst., 2, 65–71 (1969). A Profile Refinement Method for Nuclearand Magnetic Structures.] using the FullProf 98 program [J.Rodriquez-Carvajal, Abstracts of the Satellite Meeting on PowderDiffraction of the XV Congress of the IUCr, Toulouse, France. FULLPROF:A Program for Rieltveld Refinement and Pattern Matching Analysis.] (FIG.2). The results of this refinement are given in Table 1. The parametersare in good agreement with a hexagonal cell of delafossite-typestructure. The R{overscore (3)}m space group has been confirmed. Thec-axis oxygen position, which is the only refinable position, led to theexpected Cu—O and A—O distances, as shown, respectively. Overall thestructural parameters are very consistent with all previous studies inbulk materials or single crystals.

TABLE 1 Crystallographic data of the refined CuAlO₂. R{overscore (3)}m(Hexagonal Axis) Space group N° 66 Lattice a  2.861(1) Å parameters c16.937(1) Å Atomic Cu 0, 0, 0 positions Al 0, 0, ½ O 0, 0, 0.1089(2)Reliability R_(B) 8.32% factors R_(WP) 16.32%  γ² 1.73% Interatomic Cu—O1.858(3) distances Al—O 1.913(2)

Example 8

The thermal stability of CuAlO₂ was found to be stable in air until atleast 1200° C. DTA measurements do not present any notable thermal peaksduring the warming and cooling scans. The oxidation of the cuprous ionwith simultaneous formation of the Cu spinel phase according to:4CuAlO₂+½O₂→Cu₂O+2CuAl₂O₄,does not occur for T<1200° C. in contrast to high temperature ceramicproducts.

Example 9

The optical properties of these representative compounds were measured.CuAlO₂ synthesized hydrothermally (HS sample) is a black powder withoutan absorption edge in the 250–700 nm range, contrary to the standardhigh temperature solid state synthesis of the prior art (SSS sample)whereas the powder is gray with a 1.65 eV indirect bandgap found byphoto-electrochemical measurements. The visual difference in colorbetween the two samples could be associated with differences in theoxygen content. Under reducing conditions (7% H₂ in N₂), the TGA curves,as seen in FIG. 3, were obtained for each sample, and the total oxygencontent was calculated using the following reaction:CUAlO_(2+δ)+(½+δ)H₂→½Al₂O₃+Cu+(½+δ)H₂OThe CuAlO₂ HS sample has a larger total oxygen content (δ=0.185) thanthat of the SSS (δ=0.015). For instance, NiO has optical properties thatare strongly dependant on the oxygen content and provides precedent forthis phenomenon. Delafossite compounds are known to be oxygen acceptorsthat can form the super-oxide ABO_(2+δ)[R. J. Cava, W. F. Peck, J. J.Krajewski, S. W. Cheong and H. Y. Hwang, J Mater. Res., 9, 2, 314–317(1994). Electrochemical and High Pressure Superoxygenation of YCuO_(2+x)and LaCUO_(2+x) Delafossites.], and this oxidation can occur throughChimie Douce [M. Trari, J. Topfer, J. P. Doumerc, M. Pouchard, A. Ammarand P. Hagenmuller, J Solid State Chem., 111, 104–110 (1994). RoomTemperature Chemical Oxidation of Delafossite-type Oxide.]. Furthermore,a change in the electronic structure can also be due to a slight dopingwith a p-type donor, such as Na in the Al sites. Neither circumstanceappears operative here, as evidenced by the high thermal stability ofCuAlO₂ up to 1200° C. and the absence of any endo or exo thermal peaksin the DTA plots, which can be attributed to Na disappearance. Thedisappearance of the absorption edge in the 200–750 nm spectrum, or theshifting of this absorption edge into a lower wavelength regime, mayalso be attributed to particle size effects arising out of thehigh-level, good crystallinity of the sample, as seen in the SEM imagesof FIG. 4.

A combined TGA and Riteveld analysis revealed a 7.5% replacement ofmonovalent Cu with trivalent Al and intercalation of oxygen equivalentto a hole carrier doping of p=0.22. Accordingly, the superior electricalconductivity and optical behaviors of the compounds/materials describedherein can be attributed to oxygen intercalation and the high holecarrier densities available through the hydrothermal methods of thepresent invention. Comparable properties, surprising and unexpected oversyntheses and compositions of the prior art, are realized with variousother compositions available in accordance with this invention.

Example 10

The electrical properties of CuAlO₂ were also investigated asillustrative of one distinguishing aspect of this invention. Roomtemperature measurements, without corrections for specific densityproduced a conductivity σ=1.3×10⁻³S/cm. This value is at least as goodas the σ=1.7×10⁻³S/cm reported by F. A. Benko and F. P. Koffyberg [F. A.Benko and F. P. Koffyberg, J. Phys. Chem. Solids, 45, 1, 57–59, (1984).Opto-electronic Properties of CuAlO₂.]. The semi-conductive temperaturedependence is identical to the results of H. Kawazoe et al. [H. Kawazoe,M. Yasukawa, H. Hyodo, M. Kurita, H. Yanagi and H. Hosono, Nature, 389,939–942 (1997). p-Type Electrical Conduction in Transparent Thin Filmsof CuAIO₂.] made on thin films, but with an estimated activation energyof 0.093 eV for the higher temperature region (see FIG. 5).

Example 11

As discussed above, the present invention provides a general reactionscheme and methodology for the facile, hydrothermal preparation of ABO₂compounds, compositions and/or materials, including entireA_(1-m)A′_(m)B_(1-n)B′_(n)O₂ solid solutions (where 0<m<1 and 0<n<1). Asa general synthetic procedure, each starting material is provided in anamount corresponding to its stoichiometric relationship with otherreactants and/or materials. They are combined, with homogenous mixtureand grinding if needed, then placed in a Teflon reaction bag. Othersuitable reaction vessels can be utilized, as would be understood bythose skilled in the art and made aware of this invention. Likewise, thereactants/starting materials of this reaction are limited only by way ofthe stoichiometric, valence and/or chemical relationships describedherein. Commercially-available bag/pouch and bomb/autoclave equipment orstraight-forward modifications thereof can be used to prepare thecompounds of this invention in larger and/or industrial scale. Suchscale-ups, with reproducible results, are available simply by adjustingthe relative quantities of reactants/starting materials according to thestoichiometric relationships provided herein. A multitude of ABO₂phases, whether or not having delafossite structures, can be preparedaccording to the broader aspects of this invention.

For instance, the general method of this example and as specifiedelsewhere herein (example 1), can be used to prepare a wide range ofCuMO₂ (M=B=Al, Ga, Sc, In and Ln=lanthanides) compounds as well as theentire range of CuM_(1-n)M′_(n)O₂ solid solutions (where O≦n≦1). Inparticular, the entire CuAl_(1-n)Ga_(n)O₂ solid solution (O≦n≦1), nototherwise available via high temperature solid state methods of theprior art, can now be prepared hydrothermally in a single reactionsequence under facile conditions. A number of such compositions havebeen prepared (n=0, 0.2, 0.4, 0.6, 0.8 and 1.0) to vary the relativestoichiometries of Al and Ga and resulting transparency and/or p-typeconductivity. (See, also, FIGS. 6A and 6B.)

While the principles of this invention have been described in connectionwith specific embodiments, it should be understood clearly that thesedescriptions are provided only by way of example and are not intended tolimit, in any way, the scope of this invention. For instance, whilenumerous materials/compositions have been described in conjunction withvarious valency relationships and resulting delafossite-type structures,various other structural modifications are possible and available foruse as otherwise described herein as can be prepared throughstraight-forward adaptations of the present methods. Other advantagesand features of this invention will become apparent in consideration ofthe claims filed hereafter, with the scope of those claims as determinedby their reasonable equivalents, as would be understood by those skilledin the art and made aware of this invention.

1. A low-temperature and low-pressure method of preparing phase purep-type ABO₂ transparent conducting oxide compositions, said methodcomprising: providing a basic reaction medium having a divalent A-sitemetal oxide reagent, a monovalent A-site metal oxide reagent, a neutralB-site metal reagent and a trivalent B-site metal oxide reagent, saidA-site metal of said monovalent and divalent A-site metal oxide reagentsselected from the group consisting of Cu, Pt, Pd, Ag and combinationsthereof; introducing water to said reaction medium; heating said rectionmedium at a temperature substantially less than 500° C. and a pressuresubstantially less than 3000 atm; and isolating a phase pure p-type ABO₂transparent conducting oxide composition.
 2. The method of claim 1wherein said reaction medium temperature is increased.
 3. The method ofclaim 2 wherein said reaction medium temperature is about 150° C., thenincreased to about 215° C.
 4. The method of claim 1 wherein water isintroduced to said reaction medium using a water-permeable vessel. 5.The method of claim 1 wherein said ABO₂ composition has a delafossitestructure comprising a monovalent metal cation A and a trivalent metalcation B.
 6. The method of claim 5 wherein said metal cation A is Cu andsaid composition is CuBO₂.
 7. The method of claim 6 wherein said metalcation B has an ionic radius dimension between the ionic radialdimension of Al⁺³ and the ionic radial dimension of La⁺³.
 8. The methodof claim 5 wherein said metal cation A is Ag and said composition isAgBO₂.
 9. The method of claim 8 wherein said metal cation B has an ionicradius dimension between the ionic radial dimension of Al⁺³ and theionic radial dimension of La⁺³.
 10. The method of claim 5 wherein saidmetal cation A is Pd and said composition is PdBO₂.
 11. The method ofclaim 10 wherein said metal cation B has an ionic radius dimensionbetween the ionic radial dimension of Al⁺³ and the ionic radialdimension of La⁺³.
 12. The method of claim 5 wherein said metal cation Ais Pt and said composition is PtBO₂.
 13. The method of claim 12 whereinsaid metal cation B has an ionic radius dimension between the ionicradial dimension of Al⁺³ and the ionic radial dimension of La⁺³.
 14. Themethod of claim 1 wherein said composition is a polycrystallineA_(1-m)A_(m)B_(1-n)B_(n)O₂ solid solution, wherein each of A and A′ areA-site cationic metal species selected from Cu, Pd, Pt and Ag, and eachof B and B′ are B-site cationic metal species selected from Al, Ga, Sc,In and lanthanide series metals, 0<m<1 and 0<n<1.
 15. The method ofclaim 14 wherein said A-site metal is Cu and said composition isCuAl_(1-n)Ga_(n)O₂.
 16. A low-temperature and low-pressure method ofpreparing phase pure p-type ABO₂ transparent conducting oxidecompositions, said method comprising: providing a basic reaction mediumhaving a divalent A-site metal oxide reagent, a monovalent A-site metaloxide reagent, a neutral B-site metal reagent and a trivalent B-sitemetal oxide reagent, said A-site metal of said monovalent and divalentA-site metal oxide reagents selected from the group consisting of Cu,Pt, Pd, Ag and combinations thereof; introducing water to said reactionmedium heating said reaction medium at a temperature less than about250° C. and a pressure less than about 20 atm; and isolating a phasepure p-type ABO₂ transparent conducting oxide composition.
 17. Themethod of claim 16 wherein said reaction medium temperature isincreased.
 18. The method of claim 17 wherein said reaction mediumtemperature is about 150° C., then increased to about 215° C.
 19. Themethod of claim 16 wherein water is introduced to said reaction mediumusing a water-permeable vessel.
 20. The method of claim 16 wherein saidABO₂ composition has a delafossite structure comprising a monovalentmetal cation A and a trivalent metal cation B selected from the groupconsisting of metal cations having an ionic radius dimension between theionic radial dimension of A⁺³ and the ionic radial dimension of La⁺³.